Module 5 - Selecting WAN Technologies

6. 5.5 Satellite

One option for reaching the Internet when a wired broadband service is not available is to use communicational a satellite using a technology like VSAT, whereat stands for Very Small Aperture Terminal. And we can use a satellite to relay communication between different corporate locations, as you see on screen, or between corporate locations and homes or the Internet. And that's what we're going to discuss in this video. Some of the characteristics of satellite-wan communication First of all, it is going to be two-way communication. It's not like satellite TV, where you're just receiving a signal. You're going to be able to transmit with this satellite dish as well. And the very small aperture that is referred to in the VSAT name. We're talking about the diameter of the satellite dish that gets installed at our different locations. By definition, it's going to be less than 3 meters. However, it is roughly the same size as a satellite dish that you might have on your house to receive television programming. And because VSAT can work without any copper, cabling, or fiber optics running at your location, it can be a great solution for more remote locations that do not have a wired Internet connection or at least a high-speed wired Internet connection.

And another place you might see Visit is on something like an oil rig out in the middle of the Gulf of Mexico. Or maybe on a ship. For example, if you take a cruise on a cruise ship, you might be given the option to purchase access to the Internet. Did you ever wonder how that ship out in the middle of the ocean could get to the Internet? Well, it's using some variant of VSAT technology in the data rates that are possible with satellite communication. They've really improved over the years, and they now have maximum speeds in the range of about twelve megabits per second to about 100 megabits per second. But one of the drawbacks that I want you to be aware of is that because we're having to travel further up into space to get to the satellite, delays are going to be more than what we might be used to with a more traditional terrestrial internet connection. You see the satellite, we're bouncing off of it. It's in geosynchronous orbit above the Earth's equator. Specifically, it's about 22 miles above the equator. And even the light waves and radio waves, they travel very, very fast. They travel at 186 0 miles per second, or three times the speed of light in a vacuum. Suddenly, we're travelling thousands of miles.

It's over 22,000 miles to get from our location to the satellite. It's over 22,000 miles to get from the satellite back to the service provider's ground station. And then there's the delay of going out to the Internet from that ground station. So the delay starts to add up. And for a couple of years, my family and I lived in a newly constructed home where broadband was not yet available. It wasn't available from the local telephone company or the cable company. So I used this satellite technology to get on the Internet, and what I've personally noticed is a trip delay of about half a second. Now, that's not a huge deal if you're just downloading something or if you're streaming Netflix, but if you're trying to use some interactive application like Voice Over IP, this is not a great solution because there is a noticeable amount of delay when you're trying to talk back and forth with someone. And another caveat that I want you to be aware of is that this technology can be sensitive to weather conditions. You've got the satellite dish mounted on your building and a big thunderstorm rolls in. And I've seen this happen many, many times where a weather system can interrupt communication between your location and the satellite, and you have to wait for that weather condition to clear before communication can resume. However, in some circumstances, VSAT just might be the best option for getting us to another site or out to the Internet.

7. 5.6 Cable

One of the most popular technologies found in homes today for getting out to the Internet or maybe connecting back to our main office over a secure virtual private network or a VPN is cable modems. And a cable modem can use our television cable as the medium to get out to the Internet. And cable companies? They often have networks composed of both fiber optics and coaxial cable. This infrastructure is called a hybrid fiber coax or HFC distribution network. And on this network, specific frequency ranges are dedicated to specific television channels. But we can also have a frequency dedicated to transmitting data and an frequency dedicated to receiving data. In other words, upstream and downstream communications. And we usually have a wider range of frequencies for downstream communication. That's the reason that most cable modems have more bandwidth coming in than going back out to the Internet.

One downside, though, is you probably don't have an ADSL line going back to your cable company, which means you may be sharing bandwidth with your neighbors. Some people report that in the evening when most of their neighbors are at home, their internet speed drops a little bit. And there are different standards that specify what frequency ranges can be used and for what purpose. And these specifications are different Doxes versions. where DOCSIS stands for Data Over Cable Service Interface Specification. Different countries might have their own version of DOCSIS. For example, many European countries have their own standards about what frequencies can be used for data and those standards are called Euro Doxes. And the bandwidth available using cable modems has continually improved over the years. At the time of this recording, the most recent version of Doxes is Doxes 40, and it has a maximum downstream speed of ten gigabits per second and a maximum upstream speed of six gigabits per second. And that's a look at cable modems.

8. 5.7 PPP

When we're communicating between a couple of devices like routers, we have to have some sort of allayer to protocol and oftentimes on serial links, we use the point-to-point protocol, or PPP. In fact, there's even a way to do a PPP over an Ethernet connection. And the reason we might do that is to take advantage of some of the features of the point-to-point protocol. Let's consider some of those features in this video. One thing that PPP offers us is the ability to do authentication. We could do authentication using something called PAP (password authentication protocol) or Chap (handshake authentication protocol). And the strong recommendation is to use Chapin instead of PAP because PAP sends password information in clear text across the wire.

You probably do not want to do that. Something else that the point-to-point protocol can do for us is compress our data. If we were able to send more data with the same amount of bandwidth, that sounds like a really good thing. And that's what we can do with compression. We can run compression algorithms like Stacker or Predictor and it can compress the data. So if we're running low on bandwidth, we can send more data over our existing bandwidth. The trade off is running those compression algorithms. That does put a processor hit on our routers. So it might not be worth the tradeoff, but it is a feature of PPP. Another feature is the ability to detect errors that occur in transmission and then to correct those errors.

Now, personally, I'm not a big fan of this PPP feature because the way PPP accomplishes that correction is if it detects an error, then it has a duplicate copy of that packet that was corrupted and it will just use that duplicate copy. So that means we're sending a copy of all of our data, which is going to dramatically cut down on our bandwidth available. So I'm not a big fan of the way it does error detection and correction. Something I am a huge fan of is PPP's ability to do multiple links. We can have multiple physical links like serial links, and maybe a single serial link doesn't have enough bandwidth for our needs. But if we have multiple serial links, we can logically bundle those together using a protocol like MLP, short for Multiplying P2P, which is going to logically group those physical circuits into a single logical channel, which the router views as an interface with more bandwidth. It can talk over that logical channel, which is going to spread the data across those underlying physical pathways. And that's a look at a very popular two-or date link layer protocol. The point-to-point protocol.

9. 5.8 Pepo

The Point-to-Point Protocol, or PPP, is a layer two protocol, in other words, a datalink layer protocol that is oftentimes used on serial links between routers. PPP has some really cool features. And one of those features that we really like is its ability to do authentication. It can do authentication using PAP or CHAP PAP, which is a password authentication protocol. I don't recommend that one because it sends the password in clear text. The other one is much better. It's called Chap, or the Challenge Handshake authentication protocol. But we love that authentication feature of PPP so much that we oftentimes see it used in combination with Ethernet because Ethernet by itself does not have that ability to do authentication.

As a result, we overlay Ethernet with the Point of Point protocol in order to use the point-to-point protocol authentication feature, and we see Pepo, or point-to-point protocol over Ethernet, in DSL connections. Oftentimes, DSL was one of the early broadband technologies that gave people high-speed internet access from their homes. and DSL. That stands for Digital Subscriber Line. And the most popular type of DSL was called ADSL (asynchronous DSL). That allowed you to have a phone and possibly a laptop in your home while communicating over your single traditional phone line. That DSL modem and a filter could keep your phone conversation and your data separate and send it all out over a phone line. However, the DSL service providers wanted to ensure that when somebody connected a modem into their network, they were a paying customer. They wanted to do some sort of authentication and they could do that using the Point to Point protocol over Ethernet.

And if you've ever set up a DSL modem, you might remember that there's a place for a username and a password, and when that DSL modem boots up, it's going to go track to authenticate with what is called a DSLAM in the Service Providers club. A DSLAM is a DSL access multiplexer, and it's going to check those credentials that you provide from your DSL modem. The username and password, it's going to check those against an authentication server and, assuming the authentication process is successful, then the device in your home, like your laptop, can successfully communicate out to the Internet. But that's a way that the pointtopointProtocol feature of authentication was leveraged in an Ethernet environment to do authentication because Ethernet could not do authentication. And that technology is called Pepo. The point-to-point protocol over Ethernet.

10. 5.9 DSL

DSL, which stands for Digital Subscriber Line, is an abroad band technology that initially became very popular because it allowed a home user to have a high-speed internet connection using their existing telephone line. That type of DSL that was used in the home environment and often found in small businesses as well, was called ADSL, for Asymmetric DSL. The asymmetric implies that the upload and download speeds are different. And on the screen you see an example of what an ADSL connection might look like. The phone itself would probably have a filter on it, and that would filter out some of the higher frequencies used by DSL, so the user wouldn't hear the sound of data on the line. And the connection goes back to the DSL service provider and terminates on a piece of equipment called a DSLAM. That stands for DSL access multiplexer.

Basically, that's a device into which multiples subscribers connect, and then that DSLAMconnects them into the service provider's network. And the DSLAM is probably in the telephone central office, and it might be owned and operated by that telephone company. However, your DSL service provider might be a third party. And that third party may have rented floor space in the telephone company's central office. And on that floor space that they're reserving, they place their own DSLAM. That's called a "colo" or "colocation." In other words, they're collocating their equipment with the central office equipment. And there is a distance limitation of 180 feet from the DSL modem to the DSLAM. The reason is, when you have an electrical signal travelling over a telephone wire for thousands and thousands of feet, you can have some capacitance that builds up between those two wires. That's an electrical characteristic that can distort a waveform. It's called phase shift. And the telephone company for many, many years has been combating that capacitance by installing what's called a load coil. And they do that every 18 feet.

And those load coils... they add inductance to the line. And if you've studied electricity, you might remember that inductance is the opposite of capacitance. They cancel each other out. But the problem is, DSL can not cross one of those load coils. Hence the limitation. And just like you might log into a website, the DSL modem needs to log into the service provider's network. To do that, it needs to provide username and password credentials, and then they'll be authenticated by the DSL service provider's authentication server. So when you're setting up a DSL modem, enter those credentials as part of your setup. And what makes this login possible is the protocolPPPoE, which stands for point-to-point Protocol over Ethernet. And we're using Pepo so that we can leverage PPP's feature of doing authentication. And we've been talking about ADSL so far in this video, but there are several other variants of DSL, and some of those variants have different versions that run at different speeds. So there's a wide range of bandwidth that you might get from DSL.

Now, let's wrap up this video by doing a side-by-side comparison of some of the more popular DSL types. What we've been talking about is ADSL, or asymmetric DSL, implying that upload and download speeds are typically different. And we've mentioned that the maximum distance to the DSLAM is 18 ft. You might also have a symmetric DSL. This is where you have upload and download speeds that are the same. In fact, I used to work for a DSL company and this is all they offered: symmetric DSL. They marketed to businesses which had a greater need for upload speeds that might not be available with ADSL. The distance limitation to the DSLAM is a bit more restrictive, though it's 12,000 ft. and for some more speed, there's very high bitrate DSL or VDSL here. The upload and download speeds are probably different and the distance to the DSLAM is even more restrictive. It's 4000 ft. And that's a look at digital subscriber line technology.

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